This invention relates to differential amplifier apparatus.
An audio signal Va may be applied to a utilization device, such as a transducer, a processing device, such as a mixer, or a distribution device, such as a switch, in single-ended form, in which the useful signal information is represented by the voltage between a single conductor and a reference potential level, such as ground. However, it is common to distribute an audio signal associated with a video signal through a television production facility by use of two conductors on which a differential audio signal .+-. Vd is impressed at the source of the audio signal. The differential mode of signal transmission is employed because it has better noise immunity than single-ended transmission. Ambient fields and other effects induce a common mode signal component on the two conductors, so that the signal that is detected at the receiving end is a voltage Vc + Vd on one conductor and a voltage Vc - Vd on the other conductor, where Vc is the common mode signal component.
The differential signal is applied to the utilization device or other load through an interface circuit that converts the differential signal to a single-ended signal. The interface circuit must remove the common mode signal component from the signal received by the interface circuit without impairing the differential mode signal component.
It is known to use a transformer to convert a differential signal to a single-ended signal. The transformer's primary winding is connected between the two conductors, and one end of the transformer's secondary winding is connected to the inverting input of an operational amplifier. Use of a transformer has the disadvantage that its performance at low audio frequencies is poor.
Another form of interface circuit comprises an operational amplifier and an inverter that feeds the output signal of the operational amplifier to its non-inverting input. This type of interface circuit has the disadvantage that the feedback has the effect of lowering the differential mode input impedance of the operational amplifier, and in order to restore the impedance to a desirably high value it is necessary to increase resistor values, which has the effect of increasing resistance noise.
Bredemann et al, U.S. Pat. No. 4,879,521 issued Nov. 7, 1989, discloses a differential amplifier in which the common mode component of the input signal is detected and inverted and is then summed with the input signal, canceling the common mode component of the input signal.